1. Field of the Invention
The present invention involves generally the shaping of metals and, more particularly, involves an improved method for forming wide sheets and foils, especially from those metals that are difficult to process by conventional methods. The term "metal" as used herein includes elemental metals, metal alloys, oxide-dispersion strengthened alloys and intermetallic compounds unless otherwise specified.
2. Description of Related Art
Many methods are known in the art for the forming of metallic feedstock into selected shapes by solid-state deformation. These include rolling, forging, drawing or cupping, spinning, and extrusion. The usefulness of these techniques depends primarily on the feedstock metal properties such as ductility, brittleness, hardness and strain hardening.
Many metals cannot be shaped at room temperature because they are too brittle, strain harden excessively, or are too strong. Hot shaping of such a metal is often used but cooling of the metal surface by steel tools, dies, or rolls may cause cracking and thereby prevent formation of thin sections. One preferred method known in the art for forming relatively thin sheets of high-temperature metals such as titanium- and nickel-based alloys is the isothermal forming method disclosed by Metcalfe, et al. in U.S. Pat. No. 3,944,782. This method is an isothermal metal roll forging technique in which feedstock is heated and rolled under pressure by molybdenum alloy dies or rolls, which serve as conductive electrodes for passing current through the contact resistance at the roll-feedstock contact line. This contact line heating produces a "travelling hot zone" (THZ) in each molybdenum alloy roll in the work region where the work roll contacts the metal feedstock. The THZ moves along the work roll perimeter as the roll turns against the feedstock. The precision control of both temperature and pressure at the THZ in the work region causes the metal feedstock to become plastic and flow into the selected configuration. The electrical current through the work roll electrodes at the feedstock contact lines also forms a THZ in the metal feedstock within the work region where the desired plastic deformation of the feedstock metal occurs. The THZ in the feedstock differs markedly from the conditions present during conventional hot working where the feedstock is heated uniformly in a furnace before passage between unheated steel rolls that chill the feedstock surface at the work region. When I speak of the THZ in this patent, I am referring to the THZ within the feedstock work region.
The technical key to successful isothermal roll forging is the precise control of conditions within the THZ of the feedstock work region. Metals such as titanium alloys, beryllium, superalloys and titanium aluminides must be deformed in narrow ranges of temperature and pressure. Failure to control conditions precisely throughout the THZ will spoil the usefulness and value of the rolled metal product.
In U.S. Pat. Nos. 3,988,913 and 3,988,914 issued on Nov. 2, 1976, Metcalfe, et al., describe isothermal metal forming apparatus for implementing the technique disclosed in the earlier patent. In U.S. Pat. No. 4,150,279 issued on Apr. 17, 1979, Metcalfe, et al. disclose an apparatus adapted to form large metal rings using isothermal metal forming methods.
In these earlier patents, I disclosed several processes based on the isothermal metal roll forging technique. My basic concept was a method to shape feedstock between molybdenum alloy tools while precisely controlling feedstock temperature by passing an electric current through both the tools and the feedstock during the shaping process. Usually the molybdenum tools are rolls or portions of rolls used in a continuous process to perform diffusion bonding, forge welding, shape rolling, roll forging, sheet or strip rolling, or composite fabrication. The isothermal rolling of strip and sheet products is a special application of the general isothermal metal working concept. But, the molybdenum tools or rolls are not heated directly before making contact with the feedstock. To avoid roll chill, heat must flow from the electrically-heated portion of the work roll in contact with the feedstock to the adjacent portion of the work roll about to make contact with the feedstock. The time required for this heat flow limits the roll speed to about 1 to 2 inches per minute, although reductions in metal thickness exceeding 90 percent per pass can be accomplished with this method.
The isothermal metal working techniques disclosed in my earlier patents have been reduced to practice for several applications. Manufacturers now use diffusion bonding as discussed in U.S. Pat. No. 3,644,698 for manufacturing 12-foot-long T-sections of nickel-based Hastelloy X for gas turbine engine applications. Others use isothermal roll forging machines to form 0.010-inch by 2-inch titanium-based Ti6A14V alloy strips for gas turbine applications. The isothermal roll forging technique allows use of alloy compositions that were otherwise difficult to forge with the integrity necessary for gas turbine applications.
However, my earlier isothermal techniques are much more difficult to apply to the rolling of wide sheets because of the necessity of precise control of conditions within an inherently unstable deformation process occurring in a THZ over a large surface. Forming metals into wide sheets in a continuous process requires passing the feedstock through a THZ in a work region of controlled temperature and pressure such that the metal flows enough to attain a new shape, but not so much that the metal ruptures. Attempts by practitioners in the art to apply isothermal roll forging techniques to form wide sheets have proceeded over the past decade. None have succeeded in controlling THZ conditions with enough precision and uniformity to achieve useful products.
These investigators have identified several problems. Problems include difficulty with the control of the several hundred-thousand amperes of feedstock heating current necessary for wide THZ's, occasional cracking of the molybdenum alloy work roll sleeves, trapezoidal beam deformation of hollow work rolls, and slow feed rates imposed by inadequate THZ control. These problems contribute to an inability to form sheet thicknesses less than 0.050 inches in an isothermal roll forge. Until the present invention, no investigator has succeeded in solving these problems, which result in sheet product irregularity, buckling, and rupture. Consequently, continuous forming of titanium-aluminide alloy sheets wider than two inches having thicknesses less than 0.100 inches requires special methods such as pack-rolling and even then thicknesses below 0.050 inch present severe difficulties.
The key economical benefit to my original isothermal rolling methods is the single pass feature of the roll forging process. This is a substantial economical benefit enjoyed when applying the method to the manufacture of continuously rolled strip up to two inches in width and other forged components. The primary economical failure of the application of isothermal roll forging to wide sheets is the high electrical current requirement. Because the required isothermal heating current is a function of the path resistance and hence of the work roll-feedstock contact area, the typical 30,000 ampere heating current required for a two-inch roll width is increased to 180,000 amperes at 12-inches and 360,000 amperes at 24-inches of width.
Practitioners have attempted to reduce this current requirement by electrically isolating and heating a rim of the work roll surface, thereby increasing the path resistance and reducing the heating current requirement. This isolation was accomplished by introducing a "loose tire" work roll for a 12-inch mill. Investigators found that the loose tire roll did indeed reduce electrical current requirements for an isothermal rolling mill, but only by a small percentage.
However, the loose tire mill suffered from unstable THZ control because of rocking about the work roll-feedstock contact line and trapezoidal distortion of the hollow work roll tire. I later discovered that the trapezoidal distortion results from the thermal stress introduced at the heated working surface of the roll. The combination of these problems effectively prevents the precise THZ control necessary for the production of uniform thin sheet with 12-inch wide loose tire work rolls.
The necessary THZ control precision has been obtained in a two-inch isothermal rolling mill, which has produced uniform, continuous sheet using large (12 inch diameter) work rolls. This mill reduces feedstock thickness by 90 percent in a single pass, although the large reduction requires a compressive feed force applied to the feedstock to compensate for roll slippage. Also, roll chill problems and THZ control considerations require a second, independent electrical current to preheat the feedstock ahead of the work zone between the work rolls. This second electrical current requires separate, duplicate control means and circuitry. These requirements are all exacerbated by attempts to increase mill width for wider foils.
Isothermal roll forging was a distinct improvement over conventional cold and hot rolling processes and allowed the rolling of thin sheet with large rolls for the first time. This is understood by recognizing that, with isothermal roll forging, an electrical current heats the working roll through the contact resistance existent only at the line of contact with the feedstock. The thermal bulge introduced in the solid work roll by the localized heating is believed to compensate for the roll flattening normally induced by compressive stresses at the feedstock. Roll flattening is responsible for the classical assumption that thin sheet cannot be formed with large rolls.
But, when attempting to roll sheets wider than two inches, heating the roll in a local zone becomes a disadvantage because of roll chill problems arising from the thermal latency in the roll. Increasing the electrical current can overcome the thermal latency but the several hundred-thousand amperes required for significantly wider isothermal rolling mills makes the application economically infeasible. Attempts by practitioners to obtain satisfactory performance while reducing electrical current requirements by limiting current flow to the loose tire failed because trapezoidal distortion of the loose molybdenum tire resulted in increased sheet thickness at the edges, giving unsuitable results for rolled sheet.
The optimum speed of a two-inch isothermal rolling mill is about one inch per minute. This slow speed reduces the plastic flow stress in the feedstock and allows a 90 percent single-pass thickness reduction, which reduces the cost of isothermally rolled foil with respect to the cost of other conventional processes. The loose tire concept reduces the electrical current costs but allows only a 70 percent single-pass reduction, increasing the process costs accordingly. This approach also exacerbates roll sticking because of speed and flow instabilities in the THZ leading to rippling and roll slippage, and was unsuitable for rolling thin foils.
Because isothermal roll forming requires a compressive feed force, introduction of a loose tire results in roll position changes as a function of changes in compressive feed forces and exit tensile forces. The only advantage found by practitioners using a loose tire roll was the reduction in electrical current requirements and a fifty percent increase in processing speed made possible by a related reduction in thermal latency in the tire near the work region. This increase in speed was obtained at the expense of product uniformity and control precision. The minimum gauge produced by a 12-inch isothermal mill using a loose tire roll was 0.045 inches. This compares with the 0.010 inches produced by a two-inch isothermal mill using a 12-inch monolithic roll.
Roll forging means for producing thin sheets and foils from high-strength metals have been long sought in the art and are currently unknown except for the limited success of the isothermal roll forging techniques disclosed in my earlier patents. These isothermal techniques are well-suited for many applications but have limited usefulness for rolling thin sheets and foils. Attempts by practitioners in the art to apply these isothermal roll forging methods to the production of thin sheets wider than two inches have shown no signs of economic or technical feasibility. The hundreds of thousands of amperes of roll heating currents required for the wider mills using monolithic rolls are not economically feasible All methods proposed in the art for reducing this heating current expense had new and undesired effects on foil thickness, finish and uniformity.
The keenly felt need in the art for a useful technique to continuously form thin sheets or foils of difficult-to-work metals such as aluminides, intermetallics, oxide-dispersion-strengthened (ODS) composites, beryllium and others has not been met until now. The only techniques known in the art for forming these metals into useful components are of limited interest because of very high costs. The unresolved problems and deficiencies discussed above are clearly felt in the art and are solved by the present invention in the manner described below.